Note: Kraftwerk used a machine called Votrax (Siemens), its a phonetic Generator, so it is no Vocoder.. but of course they also used vocoders (a lot)..

english

originally made for coding Voice send it by wire and decode it again (milltary use) but now it is used in music. it is a Filterbank (synthesis and analysis part) with 8 or more bands and a a voiced / unvoiced detector. each band will be "scanned" by an envelope follower witch only listens to a certain range of audio. the synthsis part produces a noise signal which is filtered by (BPF) (one per band plus one HPF / LPF at the lowest / highest (or better upper and lowest) end of the band.

Voice Coder = Vocoder - analyses frequency ranges from the original and transforms it to another frequency section (vocoder=voice encoder decoder)

And most poeple who tell about vocoders basically mean "there's some tech gadget mangling the voice" but - in fact it's often a harmonizer (indie / pop music / song writer), auto tune (Mainstream Pop), or just a reverb / distortion (EBM/Industrial) - but vocoders are those sounding like ... the Kraftwerk Robot thing and are sort of a singing synthesizer - it's basically checking a few frequency bands, exchanging those bands and playing it back replacing the natural sound source by a synthesizer (mostly pads with certain amount of upper harmonics like rich string pads) to reproduce consonants and s and f noises (which can be coloured noise but as a whole it must be something tonal if you want to play melodies). so there's an analyzing part and a synthesizing part, one that transforms the microphone (or drum loop …) signal into those band filtered 8 or 16 bands (sometimes more) and so to say "sorts" out which frequency group plays a role in your stream of speach / singing.. and afterwards that synthesizer voice plays it back, maybe you have assigned those frequency bands to other bands, so this makes it sound a bit different - note - this was made to chipher encode signals - so this is just a bit simpler because you still want something that makes sense "musicially". so - here you go - the oscillator is that synth carrier signal and will play as much harmonics as that back to the designated band.. and now you have it - we are the robot'sssss... some original direct signal will help to understand what your robot is saying..

Sennheiser VSM201 Vocoder made in 1978 - this is a luxury 20bands vocoder

fully analogue of course only the "creme de la creme" used this, because it is really not cheap (kraftwerk)
manual can be found here: http://home.swipnet.se/cfmd/synths/companies/sennheiser/
as you can see: you can assign all bands freely and tune it!!
the "voice coder = vocoder" is not only for "robotic" voices like kraftwerk did. It's also interesting for rhythmic sound sources.
you won't worry about speech clearness anymore..
the vocoder was first a military device to encode speech signals.. but now "human kind" uses vocoders for music! also a good thing!
-> vsm201 frequency response